Different materials and processes are known for the manufacture of catalyst supports. High porosity and good physical strength are general requirements for such products. High temperature stability of the structure is also required if the catalyst operates at elevated temperature.
U.S. Pat. No. 5,036,037 teaches a method to produce metal oxide catalysts by pyrohydrolysis from solutions of chlorides, fluorides, or nitrates. The process makes particles with a mean size of 20-30 microns and with a high specific surface area. The processing temperature is at least 500° C. and generally higher than 700° C., to avoid the presence of the anion (chloride or fluoride) in the oxide product. The product can be used as such or can be further treated to give it the required physical or chemical properties. Although this process is suitable for the intended product, improvements to the process are desired.
Novel processes for the manufacture of titanium dioxide from aqueous solutions have been disclosed in PCT Publications WO 01/00530, WO 01/00531, and WO 01/12555, the relevant portions of which are incorporated herein by reference. In general, these applications describe the processing of an aqueous solution of a titanium salt by evaporation to produce an intermediate. The evaporation is conducted at a temperature higher than the boiling point of the solution, but lower than the temperature where significant crystal growth of an oxide phase occurs. In some embodiments, the evaporation may be conducted at a temperature higher than the boiling point of the solution but lower than the calcination temperature of the intermediate.
In the case of titanium solutions, the temperature generally ranges from 120° to 350° C., and preferably from 200° to 250° C. The process is preferably conducted by spraying, and can be accomplished in a spray dryer. The spray drying process produces thin-filmed spheres or parts of spheres, with a diameter of about 1 to 100 μm, and a shell thickness of about 0.03 to 5 μm.
After calcination and milling of these spheres or parts of spheres, and depending on the conditions of evaporation, the choice of additives, and the conditions of calcination, ultra-fine nano-sized TiO2 or, alternatively, pigment grade TiO2 can be obtained.
There has been no suggestion, however, that such a process can economically and commercially produce catalyst structures made of metal oxides from salt solutions of the metals. The present invention is therefore directed to a process to economically produce catalyst structures or catalyst supports.
Accordingly, the present invention teaches a process to produce catalysts or catalyst structures with high porosity, high specific surface area, high mechanical strength, and excellent thermal stability. In contrast to the method disclosed in U.S. Pat. No. 5,036,037, the method of the present invention uses lower temperature equipment for the first step of the process and adjunction of chemical control additives. The method uses a combination of spraying, pressing, and crystallization, which allows optimal control of the physical characteristics of the product.